Calculating the Water Costs of Water-Cooled Air Compressors


Compressed air systems are sometimes called the “4th Utility” due to their presence in almost all industrial processes and facilities. The objective of this article is to focus on the opportunity to reduce the water consumption of compressed air systems. Water consumption has leveled off in the U.S. as reductions in the power, irrigation, and industrial segments have offset increases in the public-supply segment driven by population growth. Energy managers should understand how much cooling water is required for the inventory of air compressors in their factories along with the related costs. An evaluation can then be made, of the different types of cooling systems, to ascertain water and cost reduction strategies.

Compressed air systems are present in almost all industrial processes and facilities. They have been correctly identified as an area of opportunity to reduce electrical (kW) energy costs through measures like reducing compressed air leaks and identifying artificial demand and inappropriate uses. Water-cooled air compressors can also be significant consumers of water and reducing these costs can represent a second area of opportunity.

A very “typical” industrial plant running two 125 horsepower, water-cooled, single-stage, rotary screw, air compressors can consume 11.4 million gallons of water per year. A larger installation, with a 350 horsepower rotary screw under similar circumstances, can consume 17 million gallons per year¹.

Many older facilities continue to use two-stage, water-cooled, reciprocating air compressors. Pulp and paper mills and steel mills are perfect examples. Facilities, like these, can require 550 million gallons per year of cooling water for the air compressors.

Both air compressors and compressed air dryers can be water-cooled. We highly recommend that energy managers, at multifactory corporations, take an inventory of the water-consumption of all the installed air compressors and of how the water-cooling systems function. An evaluation should be made, in each facility, of the feasibility and benefits of switching to an air-cooled air compressor or switching to a different watercooling system.

¹Figures taken from a data sheet of a single stage, lubricant cooled, rotary screw air compressors at 100 psig pressure, 8600 working hours, and 70 °F water temperature.

 

How Much Cooling Water is Required by Air Compressors?

The standard rating, for air compressor cooling water requirements, is how many gallons of water per 1,000 btu/hr is rejected into the cooling water flow. Air compressors generate a high rejection load due to their very basic inefficiency — i.e. it takes 7 to 8 input horsepower to supply 1 hp of work in compressed air. This creates a heat-of-compression generated during the process reflecting this inefficiency. Energy input not converted to work shows up as heat. This heat has to be removed for the equipment to run and for the plant to be able to use the air. Particularly today, where dry compressed air is often critical, it must be reliably and effectively after-cooled and dried to a specified pressure dew point using compressed air dryers.

Calculating the required gallons-perminute (gpm) is dependent upon several critical variables:


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